The problem of communication between a plurality of mobile stations which allows for each of the stations to communicate with all of the other stations is not new.
Such communication is especially useful in emergency situations where a group of police or firemen must be able to communicate simultaneously with all other members of the group, by voice, in order to coordinate their activities.
U. S. Pat. No. 5,274,666 describes a daisy-chain system in which each of the units in the chain receives audio signals from one other unit, attenuates the signals, adds additional audio signals, corresponding to speech by the user of the unit, to the attenuated signals and then broadcasts the composite signal to one other unit. This transmission from unit to unit continues until the chain is closed. Attenuation of the signals before rebroadcast is added to avoid looping and unwanted feedback. However, such attenuation results in variations in the audio volume depending on the position in the loop of the speaker and listener and/or an echo in the system, depending on the number of stations and the amount of attenuation. Both of these effects are undesirable and, in the extreme, can cause some of the communications to be unintelligible.
Transmission around the loop is in a frequency hopping spread spectrum mode where the receive frequency and transmit frequency of each of the units is different and where the frequencies are changed (hopped) periodically based on a pseudo-random function known to all the stations.
It is a characteristic of such a system that each of the stations is always active and transmits during a portion of each "loop" even if the particular unit has nothing to add to the conversation. Furthermore, the units must be continuously synchronized to the time and frequency of the hops as well as to the receive and transmit time of the unit, for example, by a very accurate clock in each unit.
This combination of characteristics leads to a number of inherent problems with such a system. First, noise at any of the hopping frequencies will transmit itself as noise over all of the conversations. Second, receiver or transmitter noise in any of the units causes noise to be generated in the system, even if the particular unit has nothing to say. Third, while noise considerations limit the number of units in a system, this number is still much larger than can be easily understood. In general, every listener is a potential speaker and all the speakers, could, at least in principle, speak at the same time, making speech unintelligible. There is thus no provision for listen only units which cannot speak. Finally the operation of this system results in a relatively heavy, expensive system with high power requirements and a short operating time between charges.